A Compact Differential-Mode Wide Stopband Bandpass Filter with Good and Wideband Common-Mode Suppression

This letter presents a microstrip differential-mode (DM) bandpass filter (BPF) with high and wide stopband suppression under both DM and common-mode (CM) operations. A new coupling topology is formed up to realize the DM BPF by integrating a pair of λ/2 microstrip transmission lines and two multimode resonators. The DM and CM equivalent half-circuits are established to explain the operating principal. For validation, a DM BPF operating at 2.2 GHz is implemented. Experimental results indicate that the presented DM BPF exhibits not only sharp DM passband selectivity but also deep and ultrawide stopband suppression of more than 30 dB for both DM and CM